1. Field of the Invention
The present invention relates to the measurement of defects in optical discs, and more particularly relates to the measurement of small local irregularities on the information surface of optical discs.
2. Brief Description of the Prior Art
An optical disc is a flat disc approximately the size of an LP phonograph record, made of transparent material, in which information is stored in tracks on a recording surface embedded in the interior of the disc. The tracks comprise a sequential series of tiny indicia exhibiting optical contrast with respect to the recording surface in which the tracks are formed. Information is recovered from the tracks by scanning a spot of light along the tracks and detecting the variations in the reflected light produced by the optical contrast of the indicia. Typically, the optical contrast of the indicia is exhibited as a difference in the level of reflectance of the indicia as compared with the surrounding recording surface. The optical disc has a very high density information storage capability and is used commercially for the recording and playback of video program material.
The recording surface of an optical disc must be kept quite flat to allow for error free playback of information stored on the surface. For example, a defect of even microscopic size in a video optical disc can disrupt the playback of the video program information, causing picture dropout, skipping from frame to frame, and other unacceptable playback problems.
In the manufacture of optical discs, therefore, it is desirable to have a way of detecting and measuring such defects. It is also desirable to be able to detect such defects rapidly so that defect detection can be applied economically to the disc manufacturing process.
In the field of video optical disc manufacture, defect detection has taken two forms. According to one approach, finished optical discs are taken off of the assembly line and simply played on an optical disc player apparatus. The recovered video picture is monitored, and when a video picture of unacceptable quality is observed, the disc is rejected. This approach is extremely expensive in man hours if the entire playing surface of every optical disc manufactured is examined in this way.
The second approach involves projecting one or more narrow beams of light onto the recording surface, and detecting variations in the beam of light which is reflected off of that surface. Copending U.S. patent application Ser. No. 300,364, commonly assigned to the assignee of the present invention, describes one such technique in which a conventional optical disc player apparatus is modified to provide an auxiliary beam specially arranged so that deviations in the position of the reflected beam can be detected and related to the presence and size of defects on the information surface of the optical disc.
This second approach represents an improvement over the first approach in that such a modified player apparatus can be played in a fast scan mode to inspect an optical disc for defects in a much shorter time then it takes to play the entire disc through. It is desired, however, to provide an even more rapid way of examining optical discs for defects to reduce even further the time spent in quality control inspection in the disc manufacturing process.
The present invention provides this improvement.